A point-to-point data transmission channel over a packet network that uses out-of-band signaling allows the entire channel to be used for data. This is often referred to as ‘clear channel,’ and may also be referred to here as a full-bandwidth channel. In an IP network, these channels can carry many different types of data/signaling transport applications, such as X.25, Frame Relay, ISDN, Cellular, Signaling System 7 (SS7), or other serial protocols.
Many of these data and signaling transport applications use a serial data link protocol referred to as HDLC (High-level Data Link Control). The data link layer is defined in the Open System Interconnection model as the layer providing data packet encoding and decoding, furnishing transmission protocol knowledge and management, error handling in the physical layer, flow control and frame synchronization. HDLC typically provides error handling and flow control.
In addition to HDLC itself, there are several derivatives or HDLC-related protocols. These include Link Access Procedure Balanced (LAPB), Link Access Procedure D-channel (LAPD), Synchronized Data Link Control (SDLC), as well as SS7. Many other protocols that do not rely directly upon HDLC rely upon similar framing and flagging structures.
In idle times, many HDLC and HDLC-similar protocols regenerate flags. A flag is typically a fixed byte sequence that can be repeated frequently. In a data transmission, the flag sequence typically indicates the beginning or end of a block of data, the data of which is marked for a particular purpose.
Eliminating flags before transmission on one end and regenerating them on the remote end would allow lossless compression for many types of serial protocols used over a clear channel.